What follows is another birdsmouth article. Sorry!
I know you already have several - but bear with me here.
I was the guy who built the mast in Dave Burdecki's
article. I also wrote a pair of articles
Magazine on birdsmouth (May/June and July/Aug
What is lacking in your other birdsmouth articles is
practical advice. I try to break it down to bare essentials
here, while providing some tips and tricks builders may
not think of until too late. It's (brutally!) long, but
I think you will find it easy to read and packed with practical
I didn't think any pictures or drawings of birdsmouth
were needed; there are plenty of examples in other articles.
But I did send a pic of my birdsmouth vase.
A 12-stave, elliptical birdsmouth
"spar" - of an entirely different nature:
A vase. It's birch and the staves had to be pre-bent
to take that "taper." It's also a rather dramatic
demonstration that stave width equates to "spar"
circumference (and diameter if it's round).
Building a birdsmouth
mast is easy (granted minimal tablesaw skills).
A birdsmouth mast
makes for a remarkably efficient use of a scarce and expensive
resource - wood (meaning you have to buy considerably less
Birdsmouth is harder
to describe than to do (most of what follows is in the form
of tips and jig descriptions, most of which you won't need
to bother with).
And a statement:
Birdsmouth is demanding - but only
of the very same thing you've been using all along in building
your craft anyway: Forethought.
Let's get to it:
I've built several birdsmouth masts, a birdsmouth
vase (18" high, hourglass profile, elliptical), and am
presently building birdsmouth dining room table legs. (I've
also built a number of solid wood spars - but that was before
I discovered birdsmouth.)
One problem for those who haven't tackled
the technique is simply picturing the staves and being able
to tell which dimension is what, where the birdsmouth goes,
where to take the taper, etc.
It is confusing. But no more . . .
Do this: Picture in your head a drawing of
a birdsmouth spar viewed end-on. Now cut the ring and straighten
it out (in your head) into a flat, horizontal board. The birdsmouth
faces are now all aligned (in your head) the same way, either
facing left or right (your choice).
The width of the stave is the horizontal
dimension. The width of the staves determines mast
diameter. The width of the stave is where the taper
is cut (because it is width that determines diameter),
off the square face opposite the birdsmouth cut.
The wall-thickness (or just "thickness")
is the vertical dimension, and is just what it says. The longer
the thickness dimension, the thicker the wall of the finished
Keep that picture in your head when you go
to buy your stock: a flat "board" with all the birdsmouths
facing the same way that when curled into a circle will form
a spar - because that's what you're buying. That's
exactly what you're buying - stock that will be curled
lengthwise into a circle after you cut it.
You may want to add one more thing to your
imaginary picture - grain. If the stock you buy is plain-sawn
then bending that board lengthwise into a circle will result
in grain running around your mast (just like it does in a tree),
and vice versa for quarter-sawn stock.
However, you can construct a vertical-grained
spar (a quarter-sawn mast?) from plain-sawn stock (or vice versa)
by cutting the width of the staves from the thickness of the
stock (in other words, in the picture in your head, individually
flip all the staves 90 degrees). This necessarily means that
the thickness of your stock must at least equal the
width of your staves. If it isn't, either give up on
that idea or add more staves. (Cut using this method, 1"
stock can yield a 3.75" diameter, 12-stave stick, or as
much as 5" diameter using 16 staves.)
Note: Be aware that using any number
of staves other than 8 (which uses a symmetrical birdsmouth,
45 degrees to each face) means that once the birdsmouth is
cut the longitudinal orientation of each stave is fixed (so
you can't swap a stave end-for-end).
I am not qualified to comment on which is
better in a spar, plain-sawn or quarter; maybe somebody out
there is (I'm guessing it doesn't make much difference, if any).
My preference is quarter-sawn. It hand-planes easier after glue-up
and I like the looks of the finished article better (the glue
lines tend to get hidden by all that pretty grain rising vertically
up the mast).
Now, with that imaginary picture in your head,
let's talk dimensions. They are only as complicated as you want
to make them.
The rule-of-thumb is 10 percent oversize in
diameter (of an equivalent solid wood spar), and 15 percent
of diameter wall thickness. Previous articles can give you more
exact figures for more exacting needs and specifications, but
for general purposes you won't go wrong using those numbers.
(I made my Bolger Cartopper's 12-stave birdsmouth mast the same
diameter as the solid wood version, at 20 percent wall thickness,
and haven't had a problem in four years sailing it, often in
big San Francisco Bay breezes. However I wouldn't do it that
way again; I'd stick with the rule-of-thumb.)
Example: If the specified spar is 4"
diameter and solid, make your hollow version 4.4" diameter
with a .66" wall.
(Does the thicker mast bother you? Do visions
of mast-induced eddies breaking the smooth lee-side flow over
your perfectly cut, perfectly trimmed sails and the resulting
potential speed decrease of 0.05 knots (or whatever) have you
shuddering to your core? If so, I have a suggestion: Either
reach for you wallet and buy a carbon stick, or make your birdsmouth
mast oval or elliptical - it can be done.)
There is no need to get fancy figuring stave
width unless you want to. Simply figure the mast's circumference
(diameter times 3.1416), divide by the number of staves you
want to use, round up to the nearest demarcation you're
comfortable with, say 1/16" or so, or even .1" (it's
for a pleasure craft, not the space shuttle) - and your end
result will be plenty close enough.
At some point you will probably have to cut
some scarfs because the staves are so small that they will break
at knots of any size. That's good. It'll keep your stick clear
of significant defects.
What's also good is that scarfing such small
section pieces is a breeze. Slice both pieces to be scarfed
together at the same shallow angle (10:1 is more than adequate),
at the same time, fit together and glue up. If you use epoxy
you don't even need (nor do you want) to clamp, just find a
way to hold the staves in contact with each other while keeping
them straight and aligned. You can even scarf staves that already
have the birdsmouth cut into them. Arrange the scarfs so they
are staggered up and down the length of the finished mast.
Cut all your staves on the same saw setup
so they are as identical as possible (to make setting up and
cutting your birdsmouth easier). Cut extra lengths from scrap
to use in making sure your birdsmouth cut setup is right.
Cut the birdsmouth with router or tablesaw.
It will probably be easier if you haven't introduced taper yet.
Take your time getting the set-up right. Cut the birdsmouth
to a feather edge with the sides.
On the tablesaw you can make two cuts with
a standard blade to get the birdsmouth, or one cut with a dado
blade. You'll need lots of feather boards to support and hold
down the staves if using a dado blade, and a slow feed rate.
To set the correct angle on a wobble-type dado blade find the
angle with a standard blade (measure as precisely as you can
from the face of the blade) then switch to the dado blade.
Remember, stave width determines mast diameter,
so introduce taper from the face opposite the birdsmouth. Find
the diameter on the plans at various points along the tapered
area and figure what stave width should be at those points (again,
diameter times 3.1416 divided by number of staves, and rounded
Taper can be introduced on the table saw,
with a hand plane or even a belt sander. On the table saw you'll
need a jig to do it right. With hand tools you could just eyeball
it, measuring as you go. Line up all the staves, birdsmouth
side down, and sand or plane away. Important: Keep the taper
face square! (My technique is to fashion a kind of shooting-board,
making two identical pieces of plywood that mimic stave taper,
clamping them to either side of the laid-out staves and cutting
or sanding the staves - either all at once or in multiples -
down to the plywood.)
It's easier on a table saw to cut the taper
straight. The rule of thumb (if curved is specified) is to halve
the taper. In other words, start the taper at the same point
along the mast, but halve the dimension change at the top. (If
from full size to masthead the diameter decreases by 2",
make the decrease 1" instead).
It is possible to cut curved taper on a tablesaw
(although I've never tried it). You'll need a convex taper jig
to turn the concave taper of the stave into a straight line,
and some way of securing the stave to the jig against the curve.
Curved taper is parabolic - the taper starts out easy but increases
fairly dramatically towards the end - so the last few feet or
so may prove difficult to secure to the jig, especially if you
only have 8 staves. Spars with 12, or even better 16, staves
will prove much, _much_ easier to taper using this method because
each individual stave will have significantly less taper curve
they need to be bent to in the first place, and significantly
less width in the second place (so they'll bend easier to boot).
You may find that the birdsmouth cut itself
limits the amount of taper, because you'll only be able to take
off so much width before you're cutting into the birdsmouth
(giving your stave a terminal case of split ends!). The thicker
your wall the more this becomes a potential issue, because the
thicker the wall the wider, and thus deeper, the birdsmouth
cut. However, with a 15 percent wall thickness this shouldn't
be a problem except in cases of extreme taper.
Note: Introducing spar taper solely from stave
width results in a constant wall thickness end to end in the
spar. Obviously this results in more strength - and weight -
for its diameter in the tapered sections than in the straight
sections. There is no way around this short of automated machinery
- unless! - (and I've never done this) you don't fully taper
the spar by tapering the staves, and instead introduce a portion
of the taper by planing off or sanding the finished article.
(For most applications you won't be able to taper using this
method alone because there won't be enough wall thickness to
play with.) But it strikes me you could calculate how much wall
thickness you want at various points along the taper, taper
the staves' width to a lessor amount than you otherwise would,
and introduce the rest of the taper after glue-up, thus taking
some taper out of the wall thickness. The mast would bend somewhat
easier and more evenly at the ends, which can be a good thing,
and would weigh somewhat less. Is it worth the extra effort
and minimal weight savings (albeit at the mast-head, where it
counts? I don't know; it's your stick!
After tapering the staves, test fit your spar.
Now, before you do anything else,
MAKE A PLUG. Let me repeat that - MAKE A PLUG!
Make it long enough to extend from the step
through the partner and well past all the lower fittings,
cleats and whatnot. (Yes it's extra weight, but it's down low
where it doesn't count).
If this seems like a lot of trouble, consider:
You've just become an expert in making long, hollow cylinders,
so follow through on that and make the plug birdsmouth style,
this time sized to fit inside your mast. It'll be a breeze.
Make it from any old solid wood you have laying
around. If you made an 8 stave spar the interior will be octagonal,
so you can make an octagonal plug to fit. (The distance across
the interior octagon divided by 2.4 will give you a stave width
that should fit when cleaned up.) If your spar has 12 or more
staves the interior will be close enough to round that you'll
want a round plug.
Taper the plug staves but don't taper the
plug itself. My suggestion is to take the taper, this time,
from the birdsmouth face. Just use the table saw or
a hand plane and cut a long, straight taper in the end of each
stave. When you glue-up the plug (do it before you
glue-up the mast) leave the tapered staves sticking straight
up. Inside the mast they'll serve to soften the transition from
plug to no-plug so bending stresses won't concentrate at one
Test fit the glued-up plug (don't forget to
epoxy seal the interior of the plug) in your test-fit mast and
make double sure that all the mast staves still fit together
tightly along the length of the plug. If you're using epoxy
for glue-up the plug does not need to be a tight fit.
I HIGHLY recommend epoxy for all glueing.
It's slippery when applied (a very good thing in this case),
slow setting (another very good thing in this case), it seals
the interior face (still another very good thing), it doesn't
need tight clamping (a very good thing in this case) and it
fills gaps at full strength (a very, very, VERY, VERY GOOD
Glue-up can be a one-person job. Lay all the
staves out next to one another and paint clear epoxy on three
faces (skipping the exterior face), and the exterior of the
plug. Don't skimp.
Turn all the staves birdsmouth side up. Moderately
thicken the epoxy with silica or wood flour - you won't be sanding
the stuff - and paint it on the birdsmouth faces and the exterior
of the plug. Don't skimp. If you have large mistakes, gouges
or whatnot, thicken the epoxy some more and trowel it on the
mistakes. If your plug seemed way too loose a fit, trowel the
thickened epoxy on the exterior of the plug and even on the
interior of the staves where the plug will lay.
Lay all your staves on their sides just like
in the imaginary picture in your head, all birdsmouths facing
the same way.
Place the plug on the staves where it's going
to go and roll the staves up around it. Make sure the plug is
positioned properly and put a hose clamp around the base to
hold everything in place. No need to crank down on the clamp.
Now, using waxed twine make a hand-tight spiral
wrap up the mast. The staves will more or less fall in place
as your go. The twine is your clamp.
Epoxy does not want tight clamping. Repeat
- epoxy does not want tight clamping. You just want to securely
hold everything in place, nothing more. Two or at most three
revolutions of twine per foot is all you need.
Make double-sure the staves are fully engaged
with each other in way of the plug. You may have to resort to
temporarily hose-clamping that area to squeeze out excess epoxy
between the plug and the interior wall.
Sight along it to make sure the spar is straight.
If you want the aft face of a tapered mast to describe a straight
line just lay that face down on a flat floor; the staves will
automatically align themselves into place. Take your time. The
slippery epoxy makes this job easy, and the slow set-up makes
You can plane and sand down the excess wood
or build yourself a "lathe" and use inside-out belt-sander
belts to sand it down. (If you're building more than one spar
at a time the lathe might be the way to go.)
To do that fashion a secure bench (I use a
fir timber I have laying around, set up on saw horses) and use
a variable speed drill to turn the whole spar. I've used tied-down
lawn mower wheels at each end to support the mast, with lag
bolts screwed into the spar ends as axles (with the head sawn
off the lower "axle" to chuck my drill to). Plug the
bottom few inches of the mast solid to hold that lag bolt (however,
be sure to read the last paragraph below). Put a hose clamp
around the masthead to keep the very narrow staves at that end
from splitting out when drilling and screwing in the lag bolt.
Fashion "bearings" to support the span of the spar
by drilling through 2x scrap with a hole saw, then sawing through
the hole so to make top and bottom halves, which are securely
clamped to your bench around the spar. Wax the bearing faces
for lubrication. You can offset the bearing halves as the size
of the spar diminishes as a way of keeping slop to a minimum.
Use two inside-out belt-sander belts at once, holding them opposite
each other, to equalize stress on the spinning spar as you sand
Historical fact: Back in the bad-old days
hollow spars were glued-up using non-waterproof glue and they
held up just fine, even on sea-going boats. The lesson is that
you don't need to epoxy-coat the outside of your spar. Varnish
or paint will do. (Keep it chucked in your "lathe"
and the job will go easy!)
Nor do I recommend fiberglass for the exterior,
or anywhere on it for that matter. If you are concerned about
the strength of the spar then make it fatter in the first place;
it's a much more efficient use of weight for a spar that's wood
to begin with. If it's wear you're concerned with, say from
a sprit-boom, then leather the boom.
However, and this is a big "however,"
don't seal off the inside of the spar from outside air. Go ahead
and plug the top solid. But no matter how you plug the base
make sure to leave a fat hole through it so that the moisture
that will inevitably find its way in, can find a way out.